Test corpus (empty, text, binary, compressible, incompressible) with
reference archives created by original UC2 v2.3 in DOSBox. Two CTest
tests: test_identify (magic detection) and test_extract (full
extraction pipeline verified byte-for-byte against corpus).
Restructure compat layer: #include_next headers moved to posix/ for
MinGW, new standalone headers in msvc/ for MSVC (unistd.h, utime.h,
getopt.h). Add getopt() implementation, chmod/unlink/chdir compat
functions, MSVC CRT initializer for UTF-8 console, _pgmptr fix.
Add a DJGPP CMake toolchain file and DOS compatibility layer (err.h,
fnmatch, getprogname/setprogname) so UC2 builds as a native DOS
executable via cross-compilation from Linux. The toolchain works
around a baked-in /usr/include in the DJGPP GCC binary by using -I
instead of -isystem to ensure DJGPP headers take precedence.
Add GitHub Actions CI workflow that builds and smoke-tests on both
ubuntu-latest and macos-latest.
Eremey Valetov